Electrodeposition of copper



Patented Nov. 29, 1949 ELECTRODEPOSITION OF COPPER William M. Phillips,Birmingham, and Frank L. Clifton, Detroit, Mich., assignors to GeneralMotors Corporation, Detroit, Mich., a corporation of Delaware NoDrawing.

Original application May 24, 1941,

Serial No. 395,004. Divided and this application January 17, 1947,Serial No. 722,756

4 Claims. 1

This application is a division of our copending application Serial No.395,004, filed May 24, 1941.

The invention relates to the electrodeposition of copper from acid bathsand has for its object the formation of deposits which have a greaterdegree of smoothness or brilliance than is ordinarily obtained as wellas deposits which are characterized by remarkable bufi'ability orcapability of being buffed or "colored" by the usual buflinginstrumentalities, when necessary.

The invention is based upon the use of certaln addition agents in novelrelationship and the determination of the factors affecting the actionof these agents to produce most satisfactory results.

In accordance with the present invention it is proposed to employ copperplating baths of the usual sulphuric acid type in which the copper ispresent in the form of copper sulphate. To such bath is added as aprimary addition agent a very small proportion of an organic brighteningand structure-controlling agent which may be characterized by thepresence of active" sulphur, a thiourea being a typical example. In a.standard acid copper bath containing, for example, 250 grams of coppersulphate,

CuSO4.5H2O

and 10 grams more or less of sulphuric acid per liter, the addition ofas little as 0.002 gram per liter of thiourea will bring about anoticeable improvement in brightness and structure. Much largeradditions up to above five grams per liter of thiourea have beensuccessfully employed, but the larger amounts are necessary for bestresults and therefore it is generally preferred to use in theneighborhood of 0.01 to 0.1 gram per liter.

Further improvement in the uniformity of distribution of the effect ofthe primary or activesulphur containing addition agents and in the rangeof plating conditions, such as current density, as well as thecharacteristics of the deposit may be obtained by the use of furtheradditions of what we may term control" agents. These may be of either orboth, metallic and organic non-metallic character. Of the metallic type,the metals cadmium, zinc, cobalt and nickel have somewhat similareffects. They may be added in the form of salts, for example, sulphates.Cadmium sulphate, in amounts up to about '75 grams per liter of bath,seems to enhance the brightening effect in combination with thiourea. Italso improves the deposit as to softness and texture. When thus presentin the bath the metals are deposited along the copper in smallproportions. With cadmium sulphate, for example, we have found up tofive hundredths of one percentof cadmium in the deposit.

Other salts than the sulphates may be employed if compatible with thebath and, in the case of some of the metals named, may be preferable tothe sulphates. With nickel, for example, very satisfactory results havebeen obtained by the use of nickel phosphate in amounts, for example, ofabout 35 grams per liter, a small amount of phosphoric acid being addedalso.

Of the non-metallic control agents we have found the most effective tobe organic compounds of the class generally known as surface tensionreducing or wet/ting" agents. Many of the wetting agents found suitableare aromatic sulphonates and a considerable number are on the marketunder various trade names. Among those that are useful in the acidcopper sulphate bath in combination with the primary addition agents,the following have been found especially beneficial:

Triton 720 Neomerpin Tergitol 08 From the standpoint of cost. resultsand complete compatibility with the acid copper bath the Triton 720 hasproven highly satisfactory. It is a sulphonated ether containing abranch chain and an aromatic nucleus. More specifically it is one ormore compounds of the formula wherein Ar is a hydrocarbon substitutedaromatic nucleus, M is a metal of the group consisting of sodium,potassium, calcium and magnesium, or is an ammonia or amine group, n isany whole numher from three to six and the hydrocarbon substituent ofthe aryl group has carbon atoms within the range of four to twelve.Attention is also directed to U, S. Patent 2,115.192 which disclosesthis known wetting agent. It is sold generally in the form of an aqueouspaste capable of bein poured or measured by volume and, since theamounts to be used are not critical, the paste may be added by volume,one c. c. corresponding to approximately one gram of other like agents.Tergitol 08 is described in literature and issued patents as being asodium secondary alcohol sulphate. The neomerpin compounds areunderstood to be alkyl naphthalene sulphonic acids and sodium saltsthereof.

Other organic sulphonates that have been satisfactorily used along withthiourea in the acid copper bath, particularly in combination with themetallic additions, are di-chloro-bensene sulphonic acid, nickel benzenedisulphonate. toluene sulphonic acid, naphthylamine disulphonic acid andsodium tri-isopropyl naphthalene sulphonate. These are representative ofa considerable variety of aromatic sulphonates that possess similarproperties and may be effectively employed. Where the wetting agent isadded by weight, amounts of 0.01 to grams per liter may be used, largeramounts being generally associated with higher content of the primaryaddition agent.

Further additions may be made of agents that have the function ofincreasing the bright-plating range. For example, ammonium sulphate inamounts of 1-100 grams per liter has been found eflective.

The primary addition agent above referred to as "active-sulphurcontaining" and represented typically by thiourea is not limited tothiourea alone. The class is characterized generally as containing abivalent sulphur atom both bonds of which are directly connected to acarbon atom. The thloureas having the nucleus and the isothiocyanateshaving the grouping B=C=N-- are considered representative types.

Compounds containing the nucleus characteristic of thioacetamide, andthe nucleus for example tetra methyl thiuram monosulphide. are alsoeflective brighteners.

The water soluble mercaptans, for example p-dithioglycoll andthiosalicylic acid, and even some nitrogen-containing compounds notcontaining sulphur, as a cyanamide and particularly dicyandiamide, havealso a brightening effect. The concentrations of the cyanamide anddicyandlamide are the same as for the thiourea.

It will be understood that the compound employed should be soluble inthe acid bath and should be compatible with the other constituents. Costand commercial availability will, of course, be factors in determiningthe product to be used and since thiourea, per se, CS(NH:):, is suitablefrom all standpoints, it is at present preferred for plant use.

The constitution of the bath exclusive of the novel addition agents isof standard character. For example. aqueous solutions containing 125 to250 grams per liter of the technical copper sulphate ordinarily suppliedfor plating and 5 to 100 grams of sulphuric acid are well known andthlourea is effective over the entire range with suitable adjustment asto plating conditions. However, the composition of the bath withreference to the content of copper sulphate and sulphuric acid is notcritical within operating limits.

The temperature of the bath for best results may vary from relativelylow limits, for example, 40 F., up to near 120 F. For maximum brilliancethe recommended temperature is (O-80 F.

A suitable current density is 18-48 amperes per square foot of cathodearea without agitation. This can be greatly increased and the brillianceimproved by agitation, as by movement of the P cathodes. with rates ofmovement of -25 feet 7. ing

per minute, the upper limit of current density may be readily raised to72 amperes per square foot. Even higher current densities up to aboutamperes per square foot have been successfully use The following may begiven, as examples merely. of baths from which satisfactory results haveabove are given only by way of examples as the quantities are not at allcritical. For best results we recommend, for example, 10-75 grams perliter, which addition may consist of salts oi one or more of the metalsnamed.)

Copper sulphate "grams" 250 Sulphuric acid do 10 Triton 720" c. c.- 1Thiourea grams 0.01 Water to make up one liter (This produces asemi-bright deposit which is soft and easily buffed.)

The primary addition agent is slowly depleted during the platingoperation and is to be replenished as required. The control or rangeincreasign and wetting agents, however, other than the metallic type,are apparently not depleted by plating and need replenishing only as thecomposition of the bath changes due to draaout or addition of fresh bathconstituents.

The baths described enable plated deposits of copper, or chiefly ofcopper, to be obtained which, within a certain medium range ofconditions, are truly brilliant. However, even if the conditions are notso accurately maintained as to produce an entirely brilliant deposit oruniform brightness over the entire area, still the deposits arecharacterized by extreme fineness of crystal structure and remarkableease of boiling to brilliance as compared with copper deposits from theordinary acid copper baths. The invention is therefore of especialutility in the plating of objects of irregular contour on which a brightsurface is required, as where bright deposits of nickel or chromium areto be later applied. Furthermore, by reason of the reduction in theamount and severity of the bailing operation where such may benecessary, the requirements of labor, equipment, and bufilng materialsare greatly reduced and the danger of bumng through the coating issubstantially eliminated.

We claim:

1. A bath for electrodepositing copper coman aqueous acid coppersolution comprisessentially copper sulfate and sulfuric acid and havingdissolved therein a compound selected from the group consisting ofcyanamide and dicyandiamide, said compound having a concentration insaid solution of about 0.1 gram per liter.

2. A bath for electrodepositing copper comprising an aqueous acid coppersolution comprising essentially copper sulfate and sulfuric acid andhaving dissolved therein a compound selected from the group consistingof cyanamide and dicyandiamide, said compound having a concentration insaid solution within the range 0.002 to 5 grams per liter.

3. A bath for electrodepositing copper comprising essentially coppersulphate and sulphuric acid and having dissolved therein cyanamide in aconcentration within the range of 0.002 to 6 grams per liter.

WILLIAMM. PHILLIPS. FRANK I1. CLIFTON.

REFERENCES CITED The following reierences are of record in the me oi.this patent:

Transactions 01' the Electrochemical Society. vol. 78 (1940). P ges309-316.

Special volume on Modern Electroplating. pubgramsperliter.

l5 imhed by The Electrochemical Society, 1042.

page 157. i

Hoiderman Thesis (1080) on file at Purdue Universimmefl.

